A system has been obtained in which synapses form between a clonal presynaptic element (a neuroblastoma and glioma hybrid nerve cell line) and a clonal postsynaptic element (a mouse muscle line). A biochemical technique for measuring depolarization dependent acetyl choline (ACh) release in the hybrid has been devised and acetyl choline receptor distribution on the muscle can be determined by morphological and physiological techniques. A variety of methods can be used to produce differentiation of the hybrid cells and synapse formation varies greatly with the different methods. Treatment with dibutyryl cyclic AMP (dBcAMP) increases synapse formation. Transmitter release can be facilitated by serotonin or prostaglandin PGF2 alpha. ACh receptor aggregation is increased by a factor released by the hybrid cells.